我开始学习如何为Android开发应用程序。这是我的第一个项目（测试），它一直给我同样的错误......“应用程序意外停止了”，我无法弄清楚问题出在哪里。
MainActivity.java
package com.example.layoutxml2;
>import android.os.Bundle;
import android.support.v4.app.Fragment;
import android.support.v7.app.ActionBarActivity;
import android.view.LayoutInflater;
import android.view.Menu;
import android.view.MenuItem;
import android.view.View;
import android.view.ViewGroup;
import android.widget.Button;
import android.widget.TextView;
public class MainActivity extends ActionBarActivity {
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
if (savedInstanceState == null) {
getSupportFragmentManager().beginTransaction()
.add(R.id.container, new PlaceholderFragment()).commit();
final TextView text1 = (TextView) findViewById(R.id.textView1);
Button carregar = (Button) findViewById(R.id.button1);
carregar.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
// TODO Auto-generated method stub
text1.setText("Xau Pessoal!");
}
});
}
}
@Override
public boolean onCreateOptionsMenu(Menu menu) {
// Inflate the menu; this adds items to the action bar if it is present.
getMenuInflater().inflate(R.menu.main, menu);
return true;
}
@Override
public boolean onOptionsItemSelected(MenuItem item) {
// Handle action bar item clicks here. The action bar will
// automatically handle clicks on the Home/Up button, so long
// as you specify a parent activity in AndroidManifest.xml.
int id = item.getItemId();
if (id == R.id.action_settings) {
return true;
}
return super.onOptionsItemSelected(item);
}
/**
* A placeholder fragment containing a simple view.
*/
public static class PlaceholderFragment extends Fragment {
public PlaceholderFragment() {
}
@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
Bundle savedInstanceState) {
View rootView = inflater.inflate(R.layout.fragment_main, container,
false);
return rootView;
}
}
}
fragment_main.xml
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:paddingBottom="@dimen/activity_vertical_margin"
android:paddingLeft="@dimen/activity_horizontal_margin"
android:paddingRight="@dimen/activity_horizontal_margin"
android:paddingTop="@dimen/activity_vertical_margin"
tools:context="com.example.layoutxml2.MainActivity$PlaceholderFragment"
android:background="#285178" >
<ImageView
android:id="@+id/imageView1"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_alignParentTop="true"
android:layout_centerHorizontal="true"
android:contentDescription="@string/banner"
android:src="@drawable/androidbanner" />
<TextView
android:id="@+id/textView1"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_below="@+id/imageView1"
android:layout_centerHorizontal="true"
android:layout_marginTop="20dp"
android:text="@string/saudacao"
android:textSize="26sp"
android:textColor="#ffffff"
/>
<Button
android:id="@+id/button1"
android:layout_width="fill_parent"
android:layout_height="wrap_content"
android:layout_alignLeft="@+id/imageView1"
android:layout_below="@+id/textView1"
android:layout_marginTop="32dp"
android:text="@string/carregar"
android:textColor="#ffffff"
android:onClick="selfdestruct" />
</RelativeLayout>
logcat的
06-14 16:33:35.084: E/AndroidRuntime(1628): FATAL EXCEPTION: main
06-14 16:33:35.084: E/AndroidRuntime(1628): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.layoutxml2/com.example.layoutxml2.MainActivity}: java.lang.NullPointerException
06-14 16:33:35.084: E/AndroidRuntime(1628): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1647)
06-14 16:33:35.084: E/AndroidRuntime(1628): at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:1663)
06-14 16:33:35.084: E/AndroidRuntime(1628): at android.app.ActivityThread.access$1500(ActivityThread.java:117)
06-14 16:33:35.084: E/AndroidRuntime(1628): at android.app.ActivityThread$H.handleMessage(ActivityThread.java:931)
06-14 16:33:35.084: E/AndroidRuntime(1628): at android.os.Handler.dispatchMessage(Handler.java:99)
06-14 16:33:35.084: E/AndroidRuntime(1628): at android.os.Looper.loop(Looper.java:130)
06-14 16:33:35.084: E/AndroidRuntime(1628): at android.app.ActivityThread.main(ActivityThread.java:3683)
06-14 16:33:35.084: E/AndroidRuntime(1628): at java.lang.reflect.Method.invokeNative(Native Method)
06-14 16:33:35.084: E/AndroidRuntime(1628): at java.lang.reflect.Method.invoke(Method.java:507)
06-14 16:33:35.084: E/AndroidRuntime(1628): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:839)
06-14 16:33:35.084: E/AndroidRuntime(1628): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:597)
06-14 16:33:35.084: E/AndroidRuntime(1628): at dalvik.system.NativeStart.main(Native Method)
06-14 16:33:35.084: E/AndroidRuntime(1628): Caused by: java.lang.NullPointerException
06-14 16:33:35.084: E/AndroidRuntime(1628): at com.example.layoutxml2.MainActivity.onCreate(MainActivity.java:28)
06-14 16:33:35.084: E/AndroidRuntime(1628): at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1047)
06-14 16:33:35.084: E/AndroidRuntime(1628): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1611)
06-14 16:33:35.084: E/AndroidRuntime(1628): ... 11 more
很抱歉，如果错误是显而易见的，但我真的很新，我希望改进。 感谢您的帮助 ：） 问候， 里卡多莫雷拉
答案 0 :(得分：1)
您在Fragment布局中声明了Button，但在活动中使用它们。将代码移入Fragment类进入onCreateView方法并使用rootview.findViewById（）...
package com.example.layoutxml2;
import android.os.Bundle;
import android.support.v4.app.Fragment;
import android.support.v7.app.ActionBarActivity;
import android.view.LayoutInflater;
import android.view.Menu;
import android.view.MenuItem;
import android.view.View;
import android.view.ViewGroup;
import android.widget.Button;
import android.widget.TextView;
public class MainActivity extends ActionBarActivity {
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
if (savedInstanceState == null) {
getSupportFragmentManager().beginTransaction()
.add(R.id.container, new PlaceholderFragment()).commit();
}
}
@Override
public boolean onCreateOptionsMenu(Menu menu) {
// Inflate the menu; this adds items to the action bar if it is present.
getMenuInflater().inflate(R.menu.main, menu);
return true;
}
@Override
public boolean onOptionsItemSelected(MenuItem item) {
// Handle action bar item clicks here. The action bar will
// automatically handle clicks on the Home/Up button, so long
// as you specify a parent activity in AndroidManifest.xml.
int id = item.getItemId();
if (id == R.id.action_settings) {
return true;
}
return super.onOptionsItemSelected(item);
}
/**
* A placeholder fragment containing a simple view.
*/
public static class PlaceholderFragment extends Fragment {
public PlaceholderFragment() {
}
@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
Bundle savedInstanceState) {
View rootView = inflater.inflate(R.layout.fragment_main, container,
false);
final TextView text1 = (TextView) rootView.findViewById(R.id.textView1);
Button carregar = (Button) rootView.findViewById(R.id.button1);
carregar.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
// TODO Auto-generated method stub
text1.setText("Xau Pessoal!");
}
});
return rootView;
}
}
}